Recently, the spread of an Institute of Electrical and Electronics Engineers (IEEE) 802.11g standard, an IEEE 802.11a standard, and the like, as high-speed wireless access systems (wireless local area networks (LANs)) using a band of 2.4 GHz or a band of 5 GHz have been remarkable. In these systems, an orthogonal frequency-division multiplexing (OFDM) modulation scheme, which is technology for stabilizing the performance in a multipath fading environment, is used and a maximum transmission rate of 54 M bits per second (bps) in a physical layer is realized.
On the other hand, in wired LANs, the provision of a high-speed line of 100 Mbps, such as a 100 Base-T interface of the Ethernet (registered trademark), has spread due to the spread of fiber to the home (FTTH) which uses optical fibers in individual homes, and a further increase in a transmission rate is also required in the wireless LANs.
As technology therefor, multiple input multiple output (MIMO) technology has been introduced into IEEE 802.11n as spatial multiplexing transmission technology. Furthermore, in IEEE 802.11ac, a multiuser MIMO (MU-MIMO) communication method is being studied (e.g., see Non-Patent Document 1). The MU-MIMO communication has a potential to increase the throughput in the physical layer by a factor equal to the number of transmit antennas, but a transmitting apparatus requires channel information for stations in order to obtain a transmission diversity effect using many transmit antennas. However, there is a problem in that overhead is increased due to a signal for estimating the channel information and feedback information.
FIG. 10 is a sequence diagram describing an operation of acquiring channel information of OFDM communication in accordance with the conventional art. FIG. 10 illustrates an example in which an access point AP acquires channel information for K stations (STAs) STA-1 to STA-K. K is an integer which is greater than or equal to 1. In FIG. 10, reference sign 1 represents an announce signal (null data packet announce (NDPA)) indicating that a signal for channel estimation is transmitted, reference sign 2 represents a pilot signal for estimation (null data packet (NDP)), reference signs 3-1 to 3-K represent feedback signals (channel state information feedback (CSIFB)) of channel information, and reference signs 4-2 to 4-K represent polling signals (Polling) instructing a specific communication partner to transmit a response signal.
In addition, a breakdown of the pilot signal 2 is illustrated in the upper part of FIG. 10. The pilot signal 2 includes a first pilot symbol 2-1-1, a last pilot symbol 2-1-2, and N very high throughput-long training frames (VHT-LTFs) 2-2-1 to 2-2-N for enabling channel estimation corresponding to N transmit antennas. In order to acquire channel information for 8 transmit antennas, it is necessary to transmit the VHT-LTFs 2-2-1 to 2-2-N for 8 OFDM symbols. Here, a signal Sk of a kth frequency channel of the VHT-LTFs 2-2-1 to 2-2-N can be determined as, for example, in Equations (19-11), (19-12), (19-23), and (19-24) disclosed in Non-Patent Document 2.
FIG. 11 is a block diagram illustrating a configuration of an access point (AP) 10 which acquires channel information of a wireless section of an OFDM signal in accordance with the conventional art. Reference sign 10-2 represents a long training frame generating circuit, reference sign 10-3 represents a wireless signal transmitting/receiving circuit, reference signs 10-4-1 to 10-4-N represent transmit/receive antennas, reference sign 10-5 represents a received signal demodulating circuit, reference sign 10-6 represents a feedback information extracting circuit, and reference sign 10-7 represents a channel information acquiring circuit.
When the access point (AP) 10 determines stations (STA) for which channel information is to be acquired, an announce signal (NDPA) 1 and a pilot signal (NDP) 2 in FIG. 10 are generated in the long training frame generation circuit 10-2, and conversion into analog signals, conversion into carrier frequencies, amplification, and so on are performed by the wireless signal transmitting/receiving circuit 10-3, and transmission is performed via the transmit/receive antennas 10-4-1 to 10-4-N.
When the channel information is transmitted from the stations STA-1 to STA-K by the feedback signals CSIFB 3-1 to 3-K of the channel information illustrated in FIG. 10, the access point (AP) 10 receives the signals via at least one of the transmit/receive antennas 10-4-1 to 10-4-N by using the wireless signal transmitting/receiving circuit 10-3 and outputs digital signals to the received signal demodulating circuit 10-5. The received signal demodulating circuit 10-5 establishes synchronization with the received signals, and obtains information acquired from any one of the stations STA-1 to STA-K by, for example, using channel information. The feedback information extracting circuit 10-6 extracts a feedback portion of the channel information by as feedback signal CSIFB of the channel information from the obtained demodulated bits, and the channel information acquiring circuit 10-7 acquires the channel information in each frequency channel.
Here, the fed-back channel information may be propagation channel information for a time domain, or it may be channel information in each frequency channel of OFDM, or information similar to the channel information, e.g., basis vectors obtained by applying a Gram-Schmidt orthogonalization method to the channel information, a right singular matrix of a channel information matrix, or the like can be used.
The feedback of the channel information may be compressed by representing a V matrix by angles φ and Ψ or it may be obtained by acquiring part of information of frequency channels of OFDM (e.g., see Non-Patent Document 3). When the feedback information is compressed, the channel information acquiring circuit 10-7 estimates the original channel information by decompressing or interpolating the feedback information and stores the original channel information.